Arsenic is a naturally occurring substance in the earth’s crust, and can be found normally throughout the environment. Human activities – such as pesticide use, mining, and other industrial processes – can enrich the levels of arsenic in drinking water.

Previous studies have shown that high levels of arsenic in drinking water increase the risk of bladder and lung cancer. These studies were done in populations where there was a significant episode of arsenic contamination in the community. It is still unclear, however, if long-term exposure to low concentrations of arsenic in drinking water can increase cancer risk. This is an important question because most of the drinking water in the US has low but detectable amounts of arsenic.

Researchers addressed this question by doing a large-scale comparison of arsenic concentrations in drinking water (from samples taken during the 1980s and 1990s), and cancer incidence (in 2006-2010) across 1,178 counties in the US.

Using statistical analyses, they found a positive relationship between arsenic concentration in drinking water and bladder cancer for both men and women, and lung cancer for women only. This means that counties with higher arsenic concentrations in their drinking water had a higher incidence of cancer. The researchers also showed that this relationship is not due to demographic variables such as age, race, socioeconomic status, education level, and smoking habits.

This work suggests that long term exposure to even low levels of arsenic, previously thought to be harmless, can increase the risk of bladder and lung cancer.